Can I make a boom longer by using the same front (incl the tubes of course) and just using a longer set of rear arms?
i don't see why not, a longer set of rear tubes will apply more load to the arms............ but not enough to break them as overall load is same.
I am talking say 10 - 15cm longer extension.
I guess......
Or am I dreaming............
Mark, the bending moment applied by the harness ropes on the tubes will increase as a result of the the longer boom length. The increased bending moment may or may not exceed the allowable stress in the tubes. Just try it and see if it works and be prepared for a swim if it doesn't
Sure you can, back in the 80's Windsurfing Hawaii had booms that you could get extension tubes for in all different lengths, from about 6" long to over 2' long.
The only negative was additional flex along the boom arm, the longer you go, the more flex you'll get.
The only reason, that I can think of for offering different length boom bodies was to reduce the flex. The farther back along the boom arm where the rear end enters the boom body, the less flex you'll get. Lots of flex shortens the booms, making the sail pocket deeper, but you know that.
Plus they sell more booms that way. More $$$$$ = less flex.
If you're only talking about a 5% increase in length, hard to imagine that could be catastrophic. Much more stress placed on booms from heavy sailors than a short extension. For us 65kg sailors, well washed booms never break. 
Actually, a rather heavy guy borrowed my rig a couple of years back to quickly test a board he had borrowed from WindSurfNSnow. Given that he was heavier, the sail was a little small for him, and he did quite a bit of vigorous pumping to keep the board going. The next time I used the boom, it fell apart while rigging.
Select to expand quoteHarrow said..
If you're only talking about a 5% increase in length, hard to imagine that could be catastrophic. Much more stress placed on booms from heavy sailors than a short extension. For us 65kg sailors, well washed booms never break.
Actually, a rather heavy guy borrowed my rig a couple of years back to quickly test a board he had borrowed from WindSurfNSnow. Given that he was heavier, the sail was a little small for him, and he did quite a bit of vigorous pumping to keep the board going. The next time I used the boom, it fell apart while rigging.
Bending moment is a function of the square or the span of the length of the boom. Hence a 5% increase in length results in a 25% increase in bending moment.
hi mark,i recycled a old north boom to fit a 9.5 raceboard sail that i brought, original was 205-250. i found my old gastra boom (my mum used it for a pot plant hanger) took the old double adjuster end and drilled holes in the north boom to accept the gastra adjuster arms, it now adjust out to 270 and is stiffer to use as i have done away with the shoddy north adjuster.
don't use just any alloy tube as the are not heat treated and will bend easy find old niel pride or gastra
arms the are high tensile e9 alloy
Select to expand quoteMark _australia said..
Can I make a boom longer by using the same front (incl the tubes of course) and just using a longer set of rear arms?
Just to clarify it will be carbon, exactly the same rear end but the rear end tubes will be a smidge longer.
I think it must increase stress on the front tubes, well it is clear scientific fact that it will increase it as it is a longer lever. What I wanna know is if it is safe to.
On a carbon wave boom that nobody seems to be able to break, increasing the rear tubes (that are about 600mm long) up to 750mm, doesn't sound drastic to me.
Mark, The boom in its original configuration is designed to withstand shear forces, torsion forces and bending moment associated with its largest extension from forces associated with the largest sail operating at the extreme end of its wind range. Lengthening the boom will increase the bending moment but will have little effect on the shear and torsion forces. There are a lot of variables at play including sail size, wind strength, water forces and the dynamic loads associated with sailing. Depending on the factor of safety in the original design, the extended boom could well be OK, but then again you don't really know. Give it a go. If it works, it works. If not then you may damage a perfectly good boom. Getting a definitive answer is really not possible.
Select to expand quoteMark _australia said..
Just to clarify it will be carbon, exactly the same rear end but the rear end tubes will be a smidge longer.
I think it must increase stress on the front tubes, well it is clear scientific fact that it will increase it as it is a longer lever. What I wanna know is if it is safe to.
On a carbon wave boom that nobody seems to be able to break, increasing the rear tubes (that are about 600mm long) up to 750mm, doesn't sound drastic to me.
Sorry to take the piss Mark Oztraya...
One of the tests used by engineers is called "Destructive Test". You raised the stress on the test item until it finally had enough and go "bang". That will be the final and ultimate limit that boom can bear. But please don't try it on your own boom. Borrow your mate's.
Select to expand quoteJohn340 said..
Bending moment is a function of the square or the span of the length of the boom. Hence a 5% increase in length results in a 25% increase in bending moment.
I'm curious to understand why this is the case. Why isn't it just Force x Distance?
Select to expand quoteHarrow said..
John340 said..
Bending moment is a function of the square or the span of the length of the boom. Hence a 5% increase in length results in a 25% increase in bending moment.
I'm curious to understand why this is the case. Why isn't it just Force x Distance?
Yes bending moment is force times distance. That's a measure of how much closer to the failure point you are. But flex will go up as the square because all the little bits are stressed by an extra 5% and there's 5% more of them. The square of 5% is 10%.
But a boom is a little more complex, it's not a beam supported between two simple uprights. It has a sail tensioned between the ends pulling inwards. Just as the tension in a tight rope goes through the roof with moderate load a flatter sail with too much outhaul could add a lot of bending. It might be a square function just because of that?
I believe two factors can have bigger influence on the loading characteristics.
(1). How the harness lines are connected to the boom, and
(2). The length of the inner tubes within the front section.
(1). If you have your harness line very close together, the load is concentrated on more or less a single point. So the bending moment is :
"the distance from the wishbone X the force applied by the sailor".
However, if you have your harness lines spread over quite far apart, then the bending moment will be far less because the load is now shared between the length of tubing between the harness line.
(2). The longer the end section fits within the front section, the less stress it will have on both sections. An analogy can be made of a mast extension.
Anytime that I've seen different length booms from a particular manufacturer series, the look to be built the same to me, just longer or shorter. I honestly doubt that any serious engineering calculations are done. They'd just build a prototype and see how it feels. Probably a little destructive testing done just for good measure.
^^^ yes
but when you look at a 140-190 vs a 150-200 the difference is in the front tubes not the rear.
If you could just make the back 10cm longer they would I guess. That's what makes me wonder...
As an engineer , yes u can ,just leave enough overlap of both halves , 300ml would be ample
Select to expand quoteImax said..
As an engineer , yes u can ,just leave enough overlap of both halves , 300ml would be ample
You just can't say "I'm an engineer, yes you can" and expect us to leave it at that! This is Seabreeze! Give us some numbers, a formula, a wiki link, something to chew on.
Otherwise we'll still go with Harrow.
" I honestly doubt that any serious engineering calculations are done "
Select to expand quoteHarrow said..John340 said..
Bending moment is a function of the square or the span of the length of the boom. Hence a 5% increase in length results in a 25% increase in bending moment.
I'm curious to understand why this is the case. Why isn't it just Force x Distance?
Harrow, I just checked my old shear force / bending moment diagrams for various loading cases and I was in error in my earlier statement. A 5% increase in boom length will result in a 5% increase in bending moment.
Ian , as an engineer and a heavy 120kg boom bending bastard I do understand metallurgy or carbon mallubilaty. As stated earlier longer and shorter boom use the same strength bar stock , so longer booms will be weaker than short ones all things being equal. The question was will a smige longer effect the strength , I think not as long as there is enough overlap as to not kink the structure , although there will be a slight more flex due to the length ,
Have you thought about the actual shape of the sail that you are going to use. The boom may be able to handle the extra length but will you get the right depth or draft in the sail.
Bigger sails need a bigger belly. Or a wider spread in the main body of the boom.
^^^^ yes but we are talking a few cm added to fit a small freeride on a wave boom. Like 200 up to 208 or soemthing
If I was that worried I could make the tail wider the spread the whole thing a bit
I am liking Imax's response but I have to be careful as it was the answer I wanted to hear hahahahah
Select to expand quoteMark _australia said..
^^^^ yes but we are talking a few cm added to fit a small freeride on a wave boom. Like 200 up to 208 or soemthing
If I was that worried I could make the tail wider the spread the whole thing a bit
I am liking Imax's response but I have to be careful as it was the answer I wanted to hear hahahahah
That's another good point Mark Australia. Every bit of tail extension adds preload to the flex. And as Imax points out any free play in the overlap adds non-design flex. Then a larger sail might have less draught/length = more fore/aft tension. It's all adding up. As far as fatigue/failure goes. I'm guessing John340's original rough calculation of 25% for a 5% increase in length might not be far off the mark.
FWIW the last time I had a boom fail it was using a new biggest ever sail for that particular boom. Apart from surf zone incidents how often does a boom fail using a 5.0? Never, the boom for a 5.0 is way over engineered, it's easier to make them all from the same tube stock.
some of the back ends are 10cm longer, mostly in longer booms. Ive mixed boom rears for a while as being heavier I put bend into most carbon booms. Just have to try as some booms have a stopper in them so cant push that extra 10cm in. From strength perspective the best for extending back end is north as the rear is larger diameter. Personally I dont run any boom near full extension only about 30cm max as boom flex causes alot of instability in sail (gust hits boom flexes making clew length shorter exactly what you dont want to happen), the only reason I go for longer rears is to have that 10cm more support not 10cm extra extension.
There's a much bigger issue here that no-one ever talks about.
Most boom failures are created by stress on the tube where it enters the mast grip.
Every boom I've ever owned is built so that there is no 'spreading tension' on the front half of the boom when the boom is set to its shortest length, but when the boom is at its longest, the boom is stretched open, so there is a preload of stress before you even start sailing.
Booms should be made so the front half of the boom is unstressed when the boom is set to its longest length. That means that for any length shorter, there would be an 'inward' preload instead of an 'outward' preload. Then when you were sailing, the preload would reduce the stress created by the weight of the sailor instead of adding to it.
I'd guarantee this would increase the life of your boom.
Select to expand quoteImax said..
Ian , as an engineer and a heavy 120kg boom bending bastard I do understand metallurgy or carbon mallubilaty. As stated earlier longer and shorter boom use the same strength bar stock , so longer booms will be weaker than short ones all things being equal. The question was will a smige longer effect the strength , I think not as long as there is enough overlap as to not kink the structure , although there will be a slight more flex due to the length ,
Carbon doesn't have the notion of malleability - that term is largely limited to ductile materials such as metals... ie: specifically, it is the ability to change shape without fracture.... Carbon fibre cannot change shape at all, as it is an extruded/drawn fibre.
Back to the original question - longer booms are usually heavier (so all things aren't equal)... I'd third the statement that no calculations are actually done.
If carbon booms flex ( which they do ) they have malubility , it's just not a lot. They will still bend before breaking. Everything has malubility even diamond.
Good point Harrow about the boom flexing.Mabee they do that on purpose so that there is some pretention on longer settings so as to reduce flex when using , I've noticed on longer settings that it's easier to flex the boom inward than outward , Magee because of this .?
